Not to forget the Greenhills compilers, which use a proprietary machine
description and have been ported to a variety of systems including x86
and KSR1.

FWIW (though this hardly counts as ``most''), I've heard a rumor that
SunSoft has, for several years now, tuned their schedulers using an
automated system rather like Henry Baker's ``Precise Scheduling Without
a Machine Model''. According to rumor, the scheduler executes a suite
of benchmarks, trying each benchmark a variety of scheduling options
and tuning to those options that provide the best measured performance.
They iterate through this several times across a variety of machines,
eventually selecting the option that provides the best overall speedups
without substantially hurting code quality on other Sun platforms. At
least that's the rumor I heard.